WASHINGTON (Reuters) - Flu viruses evolve freshly somewhere
in east or southeast Asia every year, spreading around the
world over the next nine months before dying out, researchers
reported on Wednesday.

Genetic analysis by two teams of international researchers
show that there are just a few initial sources of annual,
seasonal influenza epidemics. The viruses spread around the
world from these before dying.

Then every year, new strains emerge to infect people,
according to the studies published in the journals Nature and
Science.

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One team led by Edward Holmes of Pennsylvania State
University could not pinpoint the source but said that both
H3N2 and H1N1 strains of influenza appear to arise every year
from a "reservoir," perhaps in the tropics.

A second team led by Colin Russell and Derek Smith of the
University of Cambridge in Britain analyzed 13,000 samples of
H3N2 flu taken since 2002 to demonstrate this source must be in
east and southeast Asia, perhaps a different place every year.

"For over 60 years the global migration pattern of
influenza viruses has been a mystery," Russell told reporters
in a telephone briefing.

Many experts have long believed Asia, and specifically
China, to be the source of most influenza viruses.

Others hypothesized that flu viruses migrated back and
forth between the northern and southern hemispheres, or that
they cooked year-round in the tropics, to pop out every once in
a while to the rest of the world, Russell said.

"We find that viruses come out of east and southeast Asia
as a region each year and it is not any one particular country
that is the continual source of influenza viruses. So it is not
as simple as saying out of China, because out of China is not
the whole story," Russell said.

RAINY SEASONS

In tropical regions, flu tends to break out in the rainy
season. "In east and southeast Asia there is a there a lot of
variability in the timing of the rainy season and the timing of
the epidemic," Russell said.

"Bangkok and Kuala Lumpur are only 700 miles apart but they
have their flu epidemics at completely different times of
year." This means flu epidemics can be occurring almost
year-round in Asia, he said.

Then the viruses die out every year in the Americas,
Europe, Australia and the rest of Oceania, making these areas
"evolutionary graveyards," Russell said.

Even if travelers carry the flu viruses back from the
Americas to Asia, for example, people living in Asia are
already immune to those particular variants.

The World Health Organization estimates that annual
influenza epidemics infect between 5 percent and 15 percent of
the world population each year, cause 3 million to 5 million
cases of severe illness, and between 250,000 and 500,000
deaths,

About 300 million people get the flu vaccine each year.
Without it, said Smith, a person can expect to catch the flu
about once every 10 years.

Smith said the findings are important for the experts who
formulate the new flu vaccine each year. It typically includes
a cocktail of three strains, and the scientists try to predict
which strains will cause the most trouble each year.

"If we are trying to predict what will happen a year from
now we should be paying attention to what is happening in east
and southeast Asia," he said.

The researchers said their study does not have any bearing
on what might happen in a pandemic of a new source of flu, such
as the H5N1 virus now circulating mostly among birds in Asia,
Europe and Africa.